Modulation metering device



Bec. 28, 1937. H. J. SCHRADER MODULATION METERING DEVICE Filed' Jan. 28,1936 Y l l l 4 I l r l l I r l l l l JL Patented Dec. 28, 1937 UNITEDSTATES PATENT OFFICE MODULATION METERING DEVICE poration of DelawareApplication January 2s, 1936, serial No. 61,150

6 Claims.

My invention relates to modulation metering devices for radio frequencycarrier currents. More specifically, my invention is a modulation meterwhich records the amount of overmodulation of a radio transmitter duringa definite period of time.

I am aware of modulation meters which operate by the flashing of a lamp,the operation of an alarm or by operating a counter. Modulation l0devices of this type do not make a permanent record of the percentage oftime a carrier current is overmodulated. Furthermore, this type ofmodulation indicator does not indicate the amount of overmodulationoccurring during a particular period.

I propose to operate the overmodulation indicator of my invention so asto make a permanent record which will indicate the percentage of time acarrier current is overmodulated.

One of the objects of my invention is to make a permanent record whichshows the percentage of time of overmodulation of a radio transmitter.

Another object is to record the overmodulation of a radio transmitterduring predetermined in- :35 tervals.

An additional object of my invention is to provide means whereby theovermodulation of a radio transmitter will be recorded on alightsensitive paper or film.

My invention may be best understood by reference to the accompanyingdrawing, in which Figure I is a schematic diagram showing one embodimentof my modulation recorder,

Fig. II is a schematic illustration of an automatic means for moving therecording paper or lm, and

Fig. III is an illustration which shows the overmodulation effects asthey are recorded.

In Fig. I a source of modulating radio fre- 40 quency energy isrepresented by the reference numeral l. This source is Coupled to theprimary 3 of a radio frequency transformer 5. The secondary 'l of thistransformer may be tuned to resonance by a capacitor 9. An antenna Il isconnected to a tap I3 on the secondary winding .V This tuned circuit ispreferably grounded at I5. An inductor l'l is mutually coupled to thetuned antenna circuit 'l-S-l I.

One of the terminals of this inductor is connected to the grid capacitorI9. The other terminal of this capacitor is connected to the grid 2l ofa thermionic detector tube 23. The cathode 25 of this tube is connectedto the remaining terminal of the inductor l1. A grid leak resistor 2'!is connected between grid and cathode.

The anode 29 of the detector tube is connected through the primary 3l ofthe audio frequency transformer 33 to the positive terminal of a Bbattery 35. The negative terminal of the B battery is connected tocathode. A heater 3l is 5 energized by a battery 39 or any othersuitable source.

The secondary 4l of the audio frequency transformer is connected to thecathode 43 of a diode rectier 45. The anode 41 of this rectier is 10connected to the slider 49 of a potentiometer 5|. A biasing battery 53is connected across the potentiometer. The positive terminal of thisbattery is connected to one of the input terminals of a relay 55. Theother input terminal of this relay is connected to the remainingterminal of the secondary of the audio frequency transformer 4l. Aheater 5l is energized by a battery 59, or the like.

The relay which is connected to the diode rectiiier may be of anyconventional type; by way of example, a Thyratron may be used. Theoutput terminals of this relay are connected through a suitable sourceof power 6| to an illuminating device 83, which by preference is a neontube. 25 The source of power 5l is sufficient to operate the neon tube.The neon tube is located within a light-tight box 65. Within thislight-tight box is a second box 67 which is closed on all sides with asuitable diaphragm opening 69, which is 30 positioned opposite the neonlight 63.

In suitable supports at the rear of the inner box are arranged a pair ofspools 1|, '13. Around the first of these spools is rolled a suitablesupply of unexposed photographic lm or light sensitive 35 paper. Thefilm 'i5 is carried around the front and sides of the inner box 61 tothe spool 73. The photographic lm is suitably secured to this spool onwhich it is rolled after it has been exposed. The take-up spool 13 maybe connected 40 by means of a suitable shaft Tl to a take-up handle orknob 19.

By way of example, the iilm may be of the ordinary type employed insmall cameras or in place of lm a light-sensitive paper may be em- 45ployed. The lm width may be of the order of one to two inches and thediaphragm may expose a suitable section of lm of the order of one squareinch.

In the arrangement just described, a manual movement or the film iscontemplated. I have found that the iilm may be exposed for iifteenminute intervals. An exposure of this kind ordinarily providessufficient information as to the 55 percentage of time overmodulationhas occurred. However, longer or shorter intervals may be used.

In the event that automatic operation is de-' siredl I have illustrated.in Fig. II one means for automatically moving the film at predeterminedintervals. A suitable ratchet wheel di is mounted on the shaft 1l. Alever S3 is pivoted about the bearing 85. An armature 81 is pivotallymounted on the free end of the lever 83. This armature is arranged toenter the solenoid Winding 89 when the same is energized by a batteryel.

An electrical timepiece 93 has been arranged so that a contact iscompleted every fifteen minutes. This contact completes the circuit fromthe battery 9| through the solenoid winding. It is not necessary thatthe contact be continued over an appreciable time interval. The contactshould be continued long enough for the armature 3i' to be drawn withinthe solenoid winding.

It will be seen that on each downward stroke an arm S5 engages a toothof the ratchet wheel 8|. This arm' 95 is pivotally mounted at the end 91of the lever arm 83. A spring member 99 normally holds the arm against astop lill. 'Ihe spring 99 permits the arm 95 to move away from thecenter of the ratchet wheel on each downward stroke and toward thevcenter of the wheel on each upward stroke. A spring m3 is secured nearthe free end of the lever 83 to return the lever to its startingposition after every downward stroke. In the illustration, the levermechanism is shown as it is about to Vstart a downward stroke.

The operation of my invention may be briefly described as follows: Aradio frequency carrier current is supplied to the antenna. This carriercurrent is modulated by an audio frequency current. The ratio betweenthe two currents is called the percentage of modulation. If thepercentage of modulation is very low, the transmitter is not efiicientlyoperated. On the other hand, 100% modulation is the theoretical maximumlimit for high quality transmission. It is desirable to approach thislimit, but it should not be continuously exceeded. It is possible toexceed the established limit for a relatively small percentage of thetime without serious distortion. It is, therefore, desirable to providemeans for observing the percentage of time that a carrier isovermodulated.

In the circuit of Fig. 1 the combined carrier and modulation currentsare impressed on the detector 23. The detector demodulates the impressedcurrents and provides an audio frequency current corresponding to theoriginal modulation.

' This audio frequency current is impressed across the diode rectifierd5. The potentiometer in the diode circuit is used to bias the rectifierto prevent the rectification of .currents below the normal percentage ofmodulation.

As the bias is exceeded by modulation currents, or peaks thereof,greater than normal modulation, a rectified current will ow through theydiode and operate the relay or Thyratron in the diode circuit. The relayin turn completes the circuit of the power supply which illuminates theneon lamp which is housed within the photographic device.

The photographic equipment, as previously explained, includes alight-sensitive paper or film. A single peak of overmodulation isregistered on the lm with a given density. A second overmodulation peakwill increase the amount of light reaching the film and thereby furtherincrease the density of the exposure. Likewise, the greater the durationof an overmodulation peaker, the greater the exposure.

By way of example, Fig. III illustrates the effect of varying lightexposures. The sections represented by A compare with no overmodulation.The section represented by B shows slight overmodulation. The section Cis permissible overmodulation. The section D represents the upper limitof overmodulation. The section E corresponds to badly overmodulatedtransmission. Actually, the film will show exposures gradually varyingfrom light gray to black, but for purposes of illustration shaded linesare shown.

While the light-sensitive iilm may be moved manually, I prefer toautomatically advance the film every fteen minutes. Each section of thelm suchas A, B, C, D and E corresponds to iteen minute intervals ofstation operation. By examining the lm, the radio station manager mayeasily determine if the station has been properly operated at itsmaximum eiciency. It should be understood that the method describedshows the percentage of time overmodulation has occurred rather than thenumber of times overmodulation peaks occur. The percentage of time is,of course, the total time the film is in a given position to the amountof time the light is illuminated. The film must, ofcourse, be developedand the film density must be calibrated and examined to arrive at properstandards of comparison.`

Although I have shown a separate detector and rectifier, it Vshould beunderstood that the two functions may be combined in a single device. Ina similar manner, the pickup may be coupled t0 the antenna or any othersuitable point in the system. I have previously explained that the relaydevice may be any of the well known relays. Ihe photographic recordingdevice may be designed to use light-sensitive lm or paper which may bemoved in any convenient manner.

I claim as my invention1 1. In a modulation metering device, a circuitincluding a rectier, relay, and illumination means, means for impressingmodulated carrier current on said circuit, means for biasing saidrectifier to prevent rectification of currents below a predeterminedlevel, currents exceeding said bias corresponding to overmodulation, asource of power, means connecting said source to said illumination meansand to said relay Whereby the operation of the relay applies power tosaid illumination means during intervals when said modulation exceedssaid predetermined level, and light sensitive means for recording saidillumination.

2. In a device of the character of claim 1, means for adjusting the biason said rectifier to values corresponding to predetermined percentagesof modulation of said carrier currents.

3. In a device of the character of claim 1, means for intermittentlymoving said light sensitive recording means a predetermined amount atregular intervals in such manner that the density of the exposuresindicate the percentage of time said carrier current is overmodulatedduring the periods said film is stationary.

4. In a modulation metering device, a circuit 'including a rectifier,relay, and illumination means, means for impressing modulated carriercurrents on said circuit, means for biasing said rectifier to preventrectification of currents below a predetermined level, currentsexceeding said bias corresponding to overmodulation, a source of power,means connecting Said source to said illumination means and to saidrelay whereby the operation of the relay applies power to saidillumination means, and means for recording indicationsfrom which thepercentage of time said carrier current is overmodulated may bedetermined.

5. In a modulation meter of the character described, a detector, meansfor impressing a modulated carrier current on said detector, a rectiereffectively connected to the output oi said detector, means for biasingsaid rectifier whereby rectification only takes place for currentsexceeding a predetermined level, currents exceeding said biascorresponding to overmodulation, relay means associated with saidrectifier, a source of power controlled by said relay, illuminationmeans operated by said relay, and a light sensitive means for recordingsaid illumination in such manner that the sum of the overmodulationintervals for a finite period may be indicated.

6. In a modulation meter of the character described, a detector, meansfor impressing a modulated carrier current on said detector, a rectiereffectively connected to the output of said detector, means for biasingsaid rectifier whereby rectification only takes place for currentsexceeding a predetermined level, currents exceeding said biascorresponding to overmodulation, relay means associated with saidrectier, a source of power controlled by said relay, illumination meansoperated by said relay during intervals when said predetermined level isexceeded, and means for recording indications from Which the percentageof time said carrier current is overmodulated may be determined.

HAROLD J. SCHRADER.

